The invention relates to a maskless particle-beam exposure apparatus for forming a pattern on a surface of a substrate by means of a beam of energetic electrically charged particles. More in detail, the invention relates to a pattern definition means and an exposure apparatus employing this pattern definition means. In particular, the pattern definition means is a device for defining a pattern in a particle-beam exposure apparatus, which device is adapted to be irradiated with a beam of electrically charged particles and let pass the beam only through a plurality of apertures. It comprises an aperture array means which has a plurality of apertures of identical shape defining the shape of beamlets permeating said apertures, and further a blanking means to switch off the passage of selected beamlets. This blanking means has a plurality of openings, each opening corresponding to a respective aperture of the aperture array means and being provided with a deflection means controllable to deflect particles radiated through the opening off their path to an absorbing surface within said exposure apparatus.
In other words, the particle beam is generated by an illumination system which produces a homocentric or preferentially telecentric beam of energetic particles; this beam illuminates a pattern definition (PD) means having an array of apertures which can be controlled so as to allow (xe2x80x98switched onxe2x80x99) or deactivate (xe2x80x98switched offxe2x80x99) the passage of particles of the beam through the respective aperture. The beam permeates the blanking aperture array through switched-on apertures, thus forming a patterned particle beam bearing a pattern information as represented by the spatial arrangement of the apertures that are switched on. The patterned beam is then projected by means of a particle-optical projection system onto the substrate where an image of the transparent apertures is thus formed.
One important application of exposure apparatus of this kind is in the field of particle-beam lithography used in semiconductor technology, as a lithography apparatus. In order to define a desired pattern on a substrate surface, such as a circuit layer to be defined on a silicon wafer, the wafer is covered with a layer of a radiation-sensitive photoresist. Then the desired structure is imaged onto the photoresist by means of a lithography apparatus. The photoresist thus patterned is partially removed according to the pattern defined by the previous exposure step, and is now used as a mask for further structuring processes such as etching. By repeating this scheme, complicated minute structures such as an integrated circuits can be formed.
Arai et al., U.S. Pat. No. 5,369,282, discuss an electron-beam exposure system using a so-called blanking aperture array (BAA) which plays the role of the pattern definition means. The BAA carries a number of rows of apertures, and the images of the apertures are scanned over the surface of the substrate in a controlled continuous motion whose direction is perpendicular to the aperture rows. The rows are aligned with respect to each other in an interlacing manner so that the apertures form staggered lines as seen along the scanning direction. Thus, the staggered lines sweep continuous lines on the substrate surface without leaving gaps between them as they move relative to the substrate, thus covering the total area to be exposed on the substrate. In the U.S. Pat. No. 5,369,282, the apertures of every second row align and the pitch between neighboring apertures in a row is twice the width of an aperture; in general, an alignment of rows is possible based on any number n, the pitch then being n times the width of an aperture. The BAA of Arai et al., which is designed for electron radiation only, employs a complicated connection circuitry in order to address the individual apertures. Furthermore, the fact that the beam is moved over the substrate offers problems, such as imaging aberrations, with the electro-optical imaging.
The article of I. L. Berry et al. in J. Vac. Sci. Technol. B15 (1997) pp. 2382-2386, describes a PD device comprising a xe2x80x9cprogrammable aperture arrayxe2x80x9d with an array of 3000xc3x973000 apertures of 5 xcexcm side length with an n=4 alignment of rows and staggered lines. The aperture array contains additional logic circuitry, thus implementing an electronic mask scanning system in which the pattern information is passed by means of shift registers from one aperture to the next within a row. The article proposes to use a 200xc3x97 demagnification ion-optical system for imaging the apertures of the BAA onto the substrate, but does not point out how such a demagnification can be achieved.
There are several unresolved issues in the PD/BAA systems of Arai et al. and Berry et al. One of them is the damage done to the aperture array by particle irradiation endangering the lifetime of the array; this is a serious problem as the PD/BAA system contains delicate electronic circuitry. Another serious problem is the severe miniaturizing of the structures on the PD/BAA system, resulting in space scarcity.
The present invention sets out to overcome the above-mentioned shortcomings of the prior art. In particular, the basic layout of a PD device as proposed by Berry et al. shall be improved in an effective way.
This task is solved according to the invention by a pattern definition means wherein on the blanking and aperture array means, the apertures are arranged within a pattern definition field being composed of a plurality of staggered lines of apertures, wherein each of the lines comprises first segments which are free of apertures and second segments which each comprise a number of apertures spaced apart by a row offset (xe2x80x98aperture fieldsxe2x80x99); this row offset is a multiple of the width of apertures, while the length of said first segments is greater than the row offset.
The apertures are typically arranged in the pattern definition field in a regular array which, perpendicular to the direction of the lines, repeats itself every n-th line, nxe2x89xa72. This number can, for instance, be 3 or 4; also a value of 5 may be especially suitable.
In a preferred realization of the invention, the first segments of the lines are positioned adjacent to each other and form a storage field (or possible several storage fields) spanning the width of the pattern definition field. In this case, the lines may be (organizationally) ordered into groups, wherein the first segments of each group are divided along the direction of the lines into logic blocks, each of the logic blocks comprising controlling logic for a second segment of one of the lines of the group situated by the first segment.
Furthermore, the blanking means will preferably comprise buffer means for buffering information to control the deflection means associated with the apertures. These storage means are then located in the areas of the first segments.
A next aspect of the invention relates in particular to the problem to provide a PD system which, while containing a delicate (and thus expensive) circuitry, can stand a long lifetime despite the fact that it is exposed to severe irradiation from the lithography beam. This problem is met by a PD means wherein in front of the blanking means as seen in the direction of the particle beam, a cover means is provided having a plurality of openings, and each opening corresponds to a respective opening of the blanking means, the width of the openings of the cover means being smaller than the width of the openings of the blanking array means. The introduction of such a cover means removes the problem of irradiation of the blanking device but does not impede the definition of the patterned beam, in particular with respect to the shape of the beamlets and control of the pattern.
Preferably the cover means is realized as a unit other than the aperture array means. Furthermore, the aperture array means is preferably positioned after the blanking means as seen in the direction of the particle beam, and the absorbing surface (to which a beam is directed when it is deflected off its path) may be realized by a surface of the aperture array means.
In order to introduce an ion-optical correction individual to the beamlets, the aperture array means may be an aperture plate of varying thickness adapted to act as correction lenses with varying strength upon beamlets defined by the apertures. Alternatively, the aperture array means may be an aperture plate in which after each aperture an opening space is provided, said opening spaces adapted to act as correction lenses upon the respective beamlets defined by the respective apertures, said opening space further having varying width defining varying correction lens strength upon beamlets defined by the apertures.
In another advantageous variant, the aperture array means may be positioned in front of the blanking means as seen in the direction of the particle beam, and the absorbing surface is then realized by a stop plate positioned after the pattern definition means.
In a further development of the invention, the PD means further comprises a correction lens means positioned after the blanking means; such a correction lens means is, for instance useful to correct for optical defects such as a curvature of the image field. This correction lens means comprises an array of electrostatic lenses, each of said lenses acting on a beamlet projected through an opening of the blanking means. The correction lens means may be positioned after both the blanking means and the aperture array means.
A further aspect of the invention deals, in particular, with the problem of realizing a system which combines the need to produce structures of very small feature size dimensions on the substrate with the desire to provide the PD means with a complex circuitry which needs a minimum amount of space. This problem, which appears in particular in an exposure system also having a substrate holder means adapted to continuously move the substrate laterally across the patterned particle beam to expose different parts of the substrate to the patterned particle beam, and a controlling means to control switching off of apertures selected in accordance with a desired image to be produced on said substrate and the continuous motion of the substrate, is overcome by providing the projection system such that it comprises at least two consecutive demagnifying projectors, each of said projectors adapted to project the patterned beam through a cross-over into a resulting beam of reduced lateral width, wherein the first of said projectors produces an intermediate image of the transparent apertures, said image being projected by means of the following projector(s) onto said substrate surface. This composite projection system offers a high demagnificationxe2x80x94for instance 200xc3x97 or morexe2x80x94while maintaining a high level of quality of the optical system with regard to optical defects. By distributing the demagnification onto a sequence of projector stages, the total length of the projection system can be decreased substantially.
In the case that, additionally to the composite projector system, the aperture array means is further positioned in front of the blanking means as seen in the direction of the particle beam, and the absorbing surface is realized by a stop plate positioned after the pattern definition means, said stop plate can advantageously be located at the position of the cross-over of the first projector; it then has an opening corresponding to the width of the patterned beam at this position.
Another aspect of the invention addresses imaging problems which may arise from space charge effects, in particular when the amount of current that passes through the PD device varies with time. Such a temporal change of current causes temporally varying blurring and distortion of the image which is distinctly more difficult to deal with as compared to a constant charge-caused blurring. This problem is met by a method for forming a pattern on a surface of a substrate by means of a beam of energetic electrically charged particles, comprising the following steps:
producing said particle beam,
illuminating said particle beam through a PD means producing a number of beamlets using an aperture array means having a plurality of regularly arranged apertures of identical shape define the shape of said beamlets, and using a blanking means to switch off the passage of selected beamlets the others forming, as a whole, a patterned particle beam, and
projecting said patterned particle beam onto said substrate surface to form an image of those apertures which are switched on, each aperture corresponding to an image element on the substrate surface during the duration of a uniform exposure time,
wherein a subset of the apertures is switched on during a fraction of the exposure time and switched off for the remaining fraction of the exposure time, thus causing a fractional exposure at a value lower than a threshold of exposure of image elements on the substrate surface, which threshold is lower than the value corresponding to a full exposure as caused by an aperture switched on during the full exposure time.
This method according to the invention allows for the introduction of additional charge current without the need to change the structure pattern that is produced on the substrate. Apertures switched on only during a fractional time of exposure are included when for a given image pattern the corresponding current would be too low, or the corresponding current density distribution would be too inhomogenous; these apertures do lead to an irradiation of the corresponding image elements on the substrate, but not to an effective exposure as the dose of irradiation is below the threshold of exposure.
In order to level the current, it is advantageous when the number of switched-on apertures is kept constant during each fraction of exposure time, for instance by selecting an appropriate number of apertures for a fractional exposure of image elements below said threshold of exposure. In this case, it is further advantageous if the plurality of apertures is divided into groups according to a division of the area occupied by the apertures into a multitude of sub-fields, and within each group of apertures corresponding to a sub-field the number of switched-on apertures is kept constant at a set level which is uniform over these groups. Preferably, an aperture array means is used whose apertures cover 1/N of the area having apertures (i.e., the aperture fields), where N is an integer, corresponding to the row offset mentioned above divided by the width of an aperture. A relative motion between the substrate and the patterned particle beam is provided, resulting in an effective motion of the patterned particle beam over the substrate surface, the beamlets being effectively moved over a sequence of adjacent image elements on the substrate surface, wherein the image elements exposed in N subsequent positions of the patterned particle beam form a contiguous covering of a target field on the substrate.
In particular in combination with a system where the substrate is moved continuously during exposure it is suitable when the projection system comprises a deflection system, by means of which the position of the image is continuously adjusted on the substrate surface laterally in correspondence with the continuous motion of the substrate, in order to obtain a better definition of the imaged pixels. The deflection system may advantageously be adapted to adjust the position of the image according to a saw-tooth-like motion; thus, during the continuous parts (slopes) of the saw-tooth the position of the image is adjusted in a manner to hold its position relative to the substrate surface on a position of a respective target field on the substrate.